Fix the move-up portion of mom_mv_snap_to_ground

[jason-e]

One thing mom_mv_snap_to_ground 1 does is ensure the player is always at least DIST_EPSILON (0.03125 units) away from the ground. To do this, it checks to make sure there is room above the player to move them up before doing so. Sometimes, due to engine limitations, this check will appear to show that there is enough room above when there actually is not, which results in the player getting stuck after being moved up. This needs to be fixed by adding some additional safety check (assuming the underlying issue cannot be resolved).